This invention relates generally to antennas and without limitation thereto to top-loaded monopole antennas.
The monopole antenna, a vertical element fed against ground, has been used in radio (wireless) communication since the discovery of radio by Tesla and Marconi. The performance of these antennas can be improved by the addition of what is known as "top-loading". Top-loading is particularly effective in the case of electrically short antennas, that is, antennas short in height with respect to wavelength.
Top-loading can be obtained by adding capacitance at the top of an antenna. The top-loading has three major benefits. The first is that the top-loading increases the vertical current moment of the antenna, which increases the radiation resistance of the antenna to thereby increase the antenna's radiation efficiency. For electrically short monopole antennas, this radiation resistance can be theoretically increased by up to a factor of four. The top-loading additionally decreases the feed point reactance of the antenna, which decreases the feed point voltage for a given input current. This has the effect of increasing the power handling capability of the antenna. The amount of this increase is theoretically very large. A final benefit is that the increased capacitance of the top-load causes a decrease in the inherent Q factor of the antenna system, resulting in an increased antenna bandwidth.
A common top-loaded antenna using a single tower is known as an umbrella-top-loaded monopole (UTLM).
The UTLM antenna includes a single base-insulated tower usually supported by structural guy wires. In the typical UTLM antenna, the top-load is provided by sections of the guy wires that are located nearest the top of the antenna structure. These sections, known as active radials or top-load radials, are electrically connected to the antenna tower and extend radially from the tower where they terminate at a primary, high voltage insulator. Non-active sections of the guy wires then extend from the high voltage insulators to the ground and are usually connected through a series of break-up insulators to a ground anchor.
One drawback of this design is that the electric field (potential gradient) is very large at the ends of the active portion of the top-load radials located furthest from the tower. This is because the radials are the furthest separated from the shielding effect offered by the other top-load radials and are closest to the ground.
The electric-field-maximum or voltage-limit of a top-load will be reached when the top-load radials go into corona. The corona forms when the surface electric field of the wires exceeds the breakdown strength of air. Corona causes power loss and radio interference/noise.
The effects of corona, especially power loss, are proportional to frequency. For VLF and even more so for LF antennas, it is undesirable to have any portion of a utilized antenna in corona as it is known that at VLF and LF even a small amount of wire in corona can dissipate a large amount of power.
Disclosed in the referenced and incorporated Patent is a rosette shaped top-load configuration that incorporates top-load elements arranged in rhombic-shaped frames. The rhombic-shaped top-load elements originate at or near the top end of an antenna tower and extend away from the tower towards the antenna's base where, as the elements approach the end of the top-load, they come closer together and hence shield each other to thereby reduce charge density.
This self-shielding permits an antenna to operate at considerably higher voltage levels. This top-load configuration can also exhibit superior effective height and static capacitance as compared to the traditional UTLM antenna. Because of the efficiency offered by this top-load design, intrinsic bandwidth and radiated power can be superior to the typical UTLM antenna, even with the same top-load voltage limit. Such results can be achieved with a fewer number of top-load high voltage insulators than used in traditional UTLMs.
A rosette shaped top-load configuration of contiguously arranged rhombic-shaped top-load elements could require special procedures to install on towers supported by guy wires. The split rosette-shaped monopole top-load configuration described herein is designed to make these special procedures unnecessary.